This invention relates to an apparatus and method for measuring the concentration of large particles in a gas. Such particles may comprise, for example, dust, soot, solid pollutants and the like. The apparatus and method of the invention are especially intended for protecting engines and other gas receiving devices from damage caused by the intake of air containing too much dust or other particulate matter of such size as to cause engine damage. However, the instant apparatus and method have a variety of other applications; for example, they may be used for checking the efficiency of dust removal equipment used in certain environmental control systems (e.g. removal of fibers from the air in textile mills or removal of dust in grain elevators) or for monitoring environmental pollution by checking the levels of particulate materials in waste gases such as flue gases, automobile exhaust systems, cement kilns and power generation plants.
Internal combustion engines used to power tanks and other vehicles require a large supply of clean air to ensure maximum engine performance and engine life and to reduce maintenance requirements. Air cleaning systems have been developed which will remove 99% of the particulate matter which is drawn into the air intake system. Such high efficiency air cleaning systems are multi-stage units which include barrier type air filters. However, a simple dust leak in the air cleaning system (caused by, for example, accidental perforation of one of the air filters) can negate the effectiveness of the system. The vehicle operator must know when such a leak occurs so that he can shut off the engine and take steps to correct the leak problem before irreparable damage is done. This problem is especially of concern on military tanks equipped with gas turbine engines which are highly susceptible to damage by dust in the air under some operating conditions. Also, problems with excessively dusty air may be encountered in other internal combustion engines equipped with turbo chargers. A reliable dust detection system can also be valuable on any engine and on many other systems where filters are used to remove dust from the air, either to provide a supply of clean air for a particular device or area or to control dust emissions for a manufacturing operation or power generation system.
Several systems have been devised for measuring the concentration of particles in a gas. One such system is described in our U.S. Pat. No. 4,312,180 issued Jan. 26, 1982. This apparatus passes particulate-carrying gas past first and second electrically conductive members spaced apart in a first region so as to provide a potential gradient in part of the region between the first and second members of at least about 30 kilovolts per centimeter. The first and second members thus form a corona and produce ions that charge a substantial proportion of the particles in the gas. Third and fourth electrically conductive members are spaced apart in a second region downstream from the first region and an electrical potential is applied between the third and fourth members so as to provide a potential gradient of substantially less than 30 kilovolts per centimeter therebetween. The charged particles produced in the first region are attracted to either the third or fouth electrically conductive member and there release their charge so that the current flowing from the third and fourth electrically conductive members measures the concentration of particulate matter in the gas.
Another prior art apparatus for measuring the concentration of particles in a gas is described in U.S. Pat. No. 3,679,973 issued July 25, 1972 to N. S. Smith et al. In this prior art apparatus, the particulate-carrying gas is passed through a narrow throat, within which is located an ionizer which serves to charge the particles in the gas in substantially the same way as the first and second electrically conductive members in our own prior art apparatus described above. The air carrying the particles thus charged emerges from the throat into a wider duct, thereby reducing the rate of flow of the gas along the duct. The walls of the wide duct are formed of a plurality of cylindrical, electrically-conductive members insulated from one another and each connected to a separate current measuring device. The reduction in flow velocity along the duct permits the particles to move radially of the duct onto the electrically-conductive members forming the walls of the duct, where they give up their charge to the electrically-conductive members, thereby causing a current flow through each measuring device. By measuring the current flow through the various current measuring devices, the concentration and size of the particles in the gas flow can be measured. This prior art apparatus is complicated; the specific apparatus shown in the drawings of the patent requires four separate electrically-conductive members with associated insulators therebetween, four measuring devices and the voltage source necessary to charge the particles in the throat.
A further apparatus for measuring the concentration of particles in a gas is described in our co-pending application Ser. No. 490,534 of even date herewith entitled "Apparatus and Method for Measuring the Concentration of Particles in a Gas", the disclosure of which is herein incorporated by reference. This co-pending application describes apparatus having a conduit defined by electrically-conductive walls and through which the gas can flow, an electrode protruding into this conduit and insulated from the walls of the conduit and current measuring means or pulse counting means electrically connected to the electrode. The electrode and the conduit member are maintained at substantially the same potential. The flow of particulate-carrying gas along the conduit causes a current to flow from the electrode through the current measuring means or pulse counting means, the magnitude of this current (or the pulse rate therein) measuring the concentration of particles in the gas.
These three types of prior art apparatus described above all rely upon measuring electrical charge upon the particles in the gas. Other types of prior art apparatus for measuring concentration of particles in a gas include light scattering monitors and densitometer-type smoke meters.
Most of the aforementioned types of prior art apparatus for measuring the concentration of particles in a gas suffer from the disadvantage that they do not distinguish between large and small particles in the gas. Most engines and many other gas-using devices can tolerate the presence of very small particles (such as those found in many smokes) but are highly susceptible to damage by relatively large particles (typically those exceeding 5 microns in diameter). Thus, if one fits an engine or similar gas-using device with an apparatus for measuring the concentration of particles in the air entering the engine or other device and this apparatus does not distinguish between large and small particles, the particle-measuring apparatus will yield a considerable number of "false alarms" when the incoming air contains small particles which are sufficient to actuate the particle-measuring apparatus but which would not actually be harmful to the engine or other device. Such false alarms are obviously undesirable since they unnecessarily increase the down time of the engine or other device and may result in unnecessary maintenance costs. This problem of false alarms is exacerbated by the fact that the air filtration system of an engine or other device which is not harmed by very small particles will often, in order to present as little resistance to air flow as possible, be designed so that such very small particles pass the air filtration system and only the larger, potentially damaging particles are retained by the system. Furthermore, most particle-measuring systems which rely upon measuring the electrical charge upon the particles in the gas are especially susceptible to giving false alarms caused by very small particles, since small particles tend to acquire a much larger charge relative to their mass than large particles. Although the apparatus described in the aforementioned U.S. Pat. No. 3,679,973 is capable of measuring the size of particles in a gas flow, it is, as already mentioned, a highly complex apparatus, and the complicated calculations necessary to determine particle sizes using this apparatus render it entirely unsuitable for routine use in applications such as motor vehicles. Light scattering types of particle monitors do possess some capacity to distinguish different size particles, but tend to be insufficiently robust for use in applications such as motor vehicles.
It will thus be seen that there is a need for a simple, robust apparatus for measuring the concentration of large particles in a gas which can distinguish such large particles from much small particles. This invention seeks to provide such an apparatus.